Emulsification is a powerful age-old technique for mixing and dispersing immiscible components within a continuous liquid phase. Consequently, emulsions are central components of medicine, food, and performance materials. Complex emulsions, including multiple emulsions and Janus droplets, are of increasing importance in pharmaceuticals and medical diagnostics, in the fabrication of microdroplets and capsules for food, in chemical separations, for cosmetics, for dynamic optics, and chemical separations. However, quantitative detections of analytes with high sensitivity and selectivity using Janus droplets have yet to be realized. Accordingly, improved systems and methods are needed.